This invention relates generally to devices for sensing the speed of internal combustion engines. More specifically it relates to devices for sensing the revolutions per unit time of internal combustion engines.
Internal combustion engine speed sensors or tachometers are, of course, well known in the art. To the applicant's knowledge there are two methods in use at this time for this purpose. One uses a capacitive pickup while the other uses an inductive pickup electrically or magnetically coupled to the engine ignition system. In addition, it has been proposed to use various optical systems. One such system is shown in U.S. Pat. No. 3,704,405 which contemplates the provision of a perforated disk on a speedometer cable with a light source on one side of the disk and a photocell on the other side. Likewise, U.S. Pat. No. 3,348,143 suggests generally the use of magnetic, piezoelectric or optical pickup devices.
Both capacitive and inductive pickups or sensors, are susceptible in one degree or another, to unwanted and spurious signals which are strongly prevalent in a multilinear internal combustion engine.
These spurious signals in turn are passed on or conducted into the test instrument as unwanted signals. In addition, any pickup which is connected to a remote measuring or indicating device by the use of a cable conducting electrical signals encounters the problem that the connecting cable itself, since it is a capacitive device, will pick up spurious signals emanating from the high tension ignition cables of the engine.
It is an object of this invention to provide a novel apparatus for sensing signals representative of engine rpm and coupling those signals to a remote location for utilization by indicating devices.
It is a further object of this invention to provide a novel engine rpm pickup apparatus which is not sensitive to extraneous signals which might otherwise cause false indications.
A further object is to provide a means of conducting these signals to an indicating instrument without this conductor itself becoming sensitive to spurious and extraneous signals and thus passing them on to the indicating instrument.
A still further object of this invention is to provide a novel engine rpm indicating device which does not require any means for selecting the number of cylinders in the engine, and, therefore, is relatively simple and more economical to manufacture.
A further object of the invention is to provide a novel engine rpm sensing apparatus which, because it uses a minimum of components, is less susceptible to temperature variations and is, therefore, more accurate over a wide range of temperatures.
A further object of the invention is to provide a true current operated device which is not susceptible to capacitive coupling to other high voltage conductors prevalent in an internal combustion engine.